A Myosin-cleaving protease was purified to homogeneity. Under mild experimental conditions this enzyme can selectively remove the 18,000 M.W. light chain of cardiac myosin (Lc2). Our initial studies show that Lc2 may act as an inhibitor of actin-myosin interaction. This project will utilize the purified protease to study the effects of removal of the cardiac myosin Lc2 on the activity of actin-activated Mg2 ion ATPase. The studies will be done using pure actin and regulated actin as cofactors. With pure actin as cofactor we shall study the effects of Ca2 ion on the affinity of myosin for actin in the presence and absence of light chain. We shall study the mechanisms that reverse the (Lc2) inhibition of actin interaction. Attention will be focused on the effects of binding of phosphorylated and dephosphorylated Lc2 to the light chain deficient myosin in relation to measurement mentioned above. Hybridization of the Lc2 deficient cardiac myosin with the homologous light chains from skeletal, smooth muscle and molluscan myosin will be performed and the effects of Ca2 ion and/phosphorylation on the actin-myosin interaction studied. It is expected that these studies will provide unique basic information regarding the role of myosin in regulating the contraction of cardiac muscle.